Beck played the entire first half for the Redskins, who opened the preseason with a 16-7 win over Pittsburgh last week. Rex Grossman got the start in that game and connected on 19-of-26 passes for 207 yards and one touchdown.
Grossman completed 7-of-12 passes for 88 yards and an interception in the second half of Friday's game.
The two signal-callers have been engaged in a spirited camp competition to replace the departed Donovan McNabb as the Redskins' main man under center.
Rookie Roy Helu carried the ball 14 times for 101 yards for Washington. Tim Hightower rushed for 70 yards and a score on six carries. Starting running back Ryan Torain sat out again with a broken hand.
The Redskins also got a solid performance from rookie wide receiver Leonard Hankerson, who caught three passes for 46 yards.
Graham Gano added three field goals in the victory.
With Manning still recovering from offseason neck surgery, Curtis Painter got the start again for the Colts. He connected on 5-of-10 passes for 40 yards. Dan Orlovsky completed 7-of-17 passes for 67 yards and an interception.
Indianapolis, which lost 33-10 to the Rams last week, finished with only 150 total yards. Meanwhile, Washington racked up 415 yards of total offense.